It has long been known that the volume of an object, including living matter, may be measured by immersing the body in a liquid medium and measuring the quantity of liquid displaced. The ratio of the weight of the subject to its volume, as measured by the volume of the displaced liquid, determines the density of the subject. Density measurement using immersion techniques are commonly used to detect flaws or defects such as voids in various methods. The density of a material having voids will be less than the known density of the pure material.
It is also well known that the density of living matter or tissue is directly related to its fat content--the leaner the body, the greater its density. However, it is apparent that known immersion methods for measuring density are impractical and awkward when employed to measure the density or fat content of a living being since an essential step of the procedure is to immerse the subject in a liquid medium.
The immersion procedure also has drawbacks even when employed to measure the volume or density of non-living objects. For example, the observed density of porous subjects will change when immersed in a liquid because fluid fills pores and voids which are intended to be detected by the measurement. Moreover, certain materials may be damaged by immersion in liquid, and others may be physically awkward to immerse.
Other known methods and apparatus for determining density also have distinct disadvantages. U.S. Pat. No. 3,455,168 is directed to the measurement of fat content of animal tissue, and advocates placing compressed slugs of meat and fat mixtures of known volume on mechanical weighing mechanisms. Counterweights and spring compensators are used to mechanically convert the measured density to fat content, compensating for the temperature of the sample, the type of meat, and its source on the animal. However, the patent assumes that the volume of the tissue being examined is a known parameter, and thus discloses no means for measuring the volume.
U.S. Pat. No. 3,487,698 is similar to the aforementioned patent and includes automatic and sequential measurement of volume, weight and temperature of a meat sample. A computer determines the density from the volume and weight measurements, and determines a standard fat content for the specific kind of meat having that density, and then corrects the fat content for variations in the temperature of the meat. Different mechanisms are used for volume and weight measurements, and the temperature measurement is that of the meat sample and not of the ambient air.
U.S. Pat. No. 3,557,625 analyzes the fat content of a meat sample by compressing it with a piston to compute the volume, injecting probes to measure the temperature, and thereafter computing the fat content from measured parameters. The weight and volume are measured with two different apparatus. The temperature measurements are those of the meat sample, and not of the ambient air.
U.S. Pat. No. 3,769,834 discloses a method and apparatus for measuring the changes in volume of a human being under near zero gravity conditions. The subject is placed in a sealed cylinder, and a piston is used to compress the remaining gas in the cylinder by a known volume. By observing how the pressure in the chamber changes with the motion of the piston, the volume of the body can be determined. It is conceptually distinct from the method and apparatus of the invention to be disclosed herein. In the known patent, pressure is varied while temperature is held roughly constant. On the contrary, the method and apparatus of the present invention hold the pressure constant and vary the ambient temperature. Moreover, the method of volume measurement disclosed by the known patent is obviously unsuitable for wide scale application since it is carried out under near zero gravity conditions.
U.S. Pat. No. 4,144,749 discloses an example of a known method of hydrostatic measurement of the volume of a human body by immersing the subject in a tank of water, and determining the volume of the subject from the quantity of water displaced.
U.S. Pat. No. 4,144,763 discloses a volumetric apparatus for measuring body fat in which gas is compressed into a sealed chamber, the subject is placed in an adjacent chamber at room temperature, the chamber enclosing the subject is sealed, a valve connecting the two adjacent chambers is opened, and the air pressure in the adjacent chambers is measured. The volume is determined by varying the pressure of the air and not by any temperature variation thereof. The temperature is allowed to stabilize before pressure measurements are taken, and the volume is calculated from differences in pressure.
U.S. Pat. No. 4,184,371 discloses an apparatus for measuring the volume of a body by pressurizing a sealed chamber containing the body with a known displacement of air. Differentials in temperature of the ambient air are not employed in the volume measurement. The patent further discloses the application of time modulated pressure in the form of sub-sonic waves, and the use of a parallel chamber to measure volume by nulling differential pressure.
U.S. Pat. No. 4,449,406 discloses a method and apparatus used to measure the fat content of animal meats. Samples of the meat in a processing plant are first weighed, and then compressed in a piston to determine the volume, and thereafter are measured by a thermocouple to determine the temperature of the meat. The density of the sample is calculated from the weight and volume measurements, and a computer determines the fat content based upon the density. The temperature measurement of the meat is used to correct for temperature dependence in the known density/fat content relation. The temperature parameter is not used in the volume measurement.
There are several other body composition measurement techniques that have been implemented with varying degrees of success. The hydrostatic density or volume measurement is generally considered the most accurate technique available today. The most common technique is the fat fold caliper. [Jackson, A. S., and Pollock, M. L., "Generalized equations for the prediction of body composition", Medicine and Science in Sports, 10: 47, 1978.] The concept here is that fat is generally stored by the body just under the skin, partly as thermal insulation. By pinching folds of skin at various places on a body, a general idea of the body's composition can be obtained. In a similar way, taking ratios of measurements of different parts of the body can infer the amount of fat stored under the skin. [Davis, P. O., and Dotson, D. O., "Development of simplified techniques for the determination of percent body fat in adult males", The Journal of Sports Medicine and Physical Fitness, Vol. 25, pp. 255-261, 1985.]
There are references to other techniques, such as measuring the rate that infrared light is diffused by the skin. [Conway, J. M., Norris, K., and Bodwell, C. E., "A new approach for the estimation of body composition: infrared interactance", American Journal of Clinical Nutrition, Vol. 40, 1984.] A known device measures the fat content of hogs and cattle by measuring ultrasonic reflectivity.
It is apparent from the above discussion that none of the aforementioned patents or known techniques disclose a method or apparatus for calculating density and body fat using a single apparatus which simultaneously weighs the subject and calculates its volume. Moreover, none of the above patents or techniques suggest a method of calculating the volume of a human subject by varying the temperature of the ambient air (and thus varying the buoyant force exerted on the subject by the air), and thereafter calculating the volume from the buoyant force determined from the differences in the apparent weight of the subject at different ambient air temperatures. On the contrary, the methods and apparatus for calculating fat density of a human or animal subject disclosed by the aforementioned prior art patents include immersing the subject in a liquid bath; sealing the subject in a closed chamber at near zero gravity conditions; or varying air pressure in a sealed chamber.
It is the primary object of the present invention to provide an improved method and apparatus for efficiently and expeditiously determining the weight, volume and density of an object, and in particular a non-invasive method and apparatus for measuring the fat content of a living being. Other aspects and objects of the invention will be apparent from the following discussion.